Conducted EMI analysis and filter design for MHz active clamp flyback front-end converter

High frequency is the major catalyst for size reduction in the advancement of power conversion technology. It is essential to understand the EMI characteristic of high frequency converter since the EMI filter typically occupies one third of total system volume. This paper presents an insight view of CM/DM noise transformation in Flyback converter for low power adapter applications. The so-called mixed-mode noise is DM noise transformed from CM noise due to unbalanced impedance when the diode-bridge is off. The transformer shielding technique can effectively reduce the CM noise, and therefore it can significantly reduce the magnitude of CM transformed noise. A 65W (19V/3.3A) prototype of active clamp flyback front-end converter operating around 1MHz with GaN devices is developed to verify the analysis. A single stage EMI filter is designed accordingly with much smaller size compared with industry practice.